From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932459AbcBCBwK (ORCPT ); Tue, 2 Feb 2016 20:52:10 -0500 Received: from mail-lb0-f193.google.com ([209.85.217.193]:33957 "EHLO mail-lb0-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932130AbcBCBwI (ORCPT ); Tue, 2 Feb 2016 20:52:08 -0500 MIME-Version: 1.0 In-Reply-To: <56B158A0.6060604@codeaurora.org> References: <5a012e56b8404e2d07e172b6699ce02f5c5b5f26.1454410226.git.viresh.kumar@linaro.org> <20160202154717.GI3947@e106622-lin> <20160202170144.GL3947@e106622-lin> <56B12BEC.9070603@codeaurora.org> <56B158A0.6060604@codeaurora.org> Date: Wed, 3 Feb 2016 02:52:06 +0100 X-Google-Sender-Auth: avPk-lUfK3yt6K-K5aA9RtU4U_4 Message-ID: Subject: Re: [PATCH 2/5] cpufreq: governor: Create separate sysfs-ops From: "Rafael J. Wysocki" To: Saravana Kannan Cc: "Rafael J. Wysocki" , Viresh Kumar , Juri Lelli , Rafael Wysocki , Lists linaro-kernel , "linux-pm@vger.kernel.org" , Peter Zijlstra , Michael Turquette , Steve Muckle , Vincent Guittot , Morten Rasmussen , dietmar.eggemann@arm.com, Linux Kernel Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 3, 2016 at 2:32 AM, Saravana Kannan wrote: > On 02/02/2016 05:07 PM, Rafael J. Wysocki wrote: >> >> On Wed, Feb 3, 2016 at 12:42 AM, Rafael J. Wysocki >> wrote: >>> >>> On Tue, Feb 2, 2016 at 11:21 PM, Saravana Kannan >>> wrote: >>>> >>>> On 02/02/2016 11:40 AM, Rafael J. Wysocki wrote: >>>>> >>>>> >>>>> On Tue, Feb 2, 2016 at 6:01 PM, Juri Lelli wrote: >>>>>> >>>>>> >> >> [cut] >> >>>> >>>> I also don't like this patch because it forces governors to either >>>> implement >>>> their own macros and management of their attributes or force them to use >>>> the >>>> governor structs that come with cpufreq_governor.h. cpufreq_governor.h >>>> IMHO >>>> is very ondemand and conservative governor specific and is very >>>> irrelevant >>>> for sched-dvfs or any other governors (hint hint). >>>> >>>> The only time this ABBA locking is an issue is when governor are >>>> changing >>>> and trying to add/remove attributes. That can easily be checked in >>>> store_governor and dealt with without holding the policy rwsem if the >>>> governors can provide their per sys and per policy attribute arrays as >>>> part >>>> of registering themselves. >>>> >>>> I'm sorry that I just keep talking about the idea and not sending out >>>> the >>>> patches. >>> >>> >>> I think you have a point, though. >>> >>> The deadlock really is specific to the governors using the code in >>> cpufreq_governor.c. >> >> >> That said no other governors in the tree use any sysfs attributes for >> tunables AFAICS and the out-of-the tree ones are out of interest here. > > > But if we are expecting sched dvfs to come in, why make it worse for it. It > would be completely pointless to try and shoehorn sched dvfs to use > cpufreq_governor.c Well, do you honestly think that using the existing stuff in it would be a good idea? If not, then why it matters at all? >> Also the deadlock happens if one of the tunable attributes is accessed >> while we're trying to remove it which very well may happen on read >> access too. > > Isn't this THE deadlock we are talking about? The removal of the attributes > only happen when governors are changes and we send a POLICY_EXIT and or all > the cores are hotplugged out. It generally happens when the "old" governor is going away, whatever the reason. > And my suggestion would work just as well there. > > Why are you prefixing your sentence with "Also"? Is there some other case > I'm not considering? Say someone is reading sampling_rate for a policy with 1 CPU in it and someone else is taking the CPU offline. The governor EXIT code path (that will trigger as a result) will try to remove the sampling_rate attribute and (if it does that under policy->rwsem) it'll wait for the read access to finish. Where exactly would you put the deadlock prevention in this case? Thanks, Rafael